NOVEL AMORPHOUS AND NANOSTRUCTURED Nb-Zr ALLOY

ABSTRACT

A method for preparing an alloy, including mixing elemental powders of Nb and Zr to obtain a powder mixture, and mechanical alloying to obtain an alloyed powder mixture, where the alloyed powder is more than 50% amorphous. The spark plasma sintering machine was used for the consolidation of the Nb 60 Zr 40  sample prepared by mechanical alloying. An Nb—Zr alloy composition obtained by the method, having an average crystallite size of 18 to 26 nm, hardness of 580.

BACKGROUND OF THE INVENTION

1. Field of the Disclosure

The present invention relates to a Nb—Zr alloy composition and a method for preparing the alloy, comprising mixing elemental powders of Nb and Zr to obtain a powder mixture, and mechanical alloying in a ball mill to obtain an alloyed powder, where the alloyed powder is more than 50% amorphous, and synthesis of nanostructured Nb—Zr alloy in a bulk form by spark plasma sintering from the milled powder.

2. Description of Related Art

The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present disclosure.

Amorphous alloys or metallic glasses have a combination of physical, chemical, mechanical, and magnetic properties and are used in several applications (Rapidly Solidified Alloys, edited by H. H. Liebermann (Marcel Dekker, New York, 1993), T. R. Anantharaman and C. Suryanarayana, Rapidly Solidified Metals: A Technological Overview (Trans Tech Publications, Zurich, 1987), Glassy Metals III, edited by H. Beck and H.-J. Güntherodt (Springer-Verlag, Berlin, 1994), C. Suryanarayana and A. Inoue, Bulk Metallic Glasses (CRC Press, Boca Raton, Fla., 2011)—each incorporated by reference in their entirety). These alloys have been produced in alloy systems by non-equilibrium processing methods such as rapid solidification processing of liquid alloys involving solidification rates of as high as 10⁶ K/s, vapor deposition (where the quenching rates can be even higher, typically 10¹² K/s), mechanical alloying, ion beam mixing, and others (D. M. Mattox, Handbook of Physical Vapor Deposition (PVD) Processing (Noyes Publications, Westwood, N J, 1998), C. Suryanarayana, Prog. Mater. Sci. 46, 1-184 (2001), C. Suryanarayana, Mechanical Alloying and Milling (Marcel Dekker, New York, 2004), B. X. Liu, W. S. Lai, and Z. J. Zhang, Adv. Phys. 50, 367-429 (2001), C. Suryanarayana, Non-Equilibrium Processing of Materials (Pergamon, Oxford, 1999)—each incorporated by reference in their entirety). The majority of these metastable alloys have been produced by liquid quenching or vapor quenching methods in alloy systems that show a negative heat of mixing, since, only under this condition, the constituent elements mix thoroughly in the liquid or vapor phases. But, when the heat of mixing is positive, constituent elements cluster together and no alloying occurs.

Consequently, there have not been reports of amorphous phase formation in alloy systems that show a positive heat of mixing. However, alloys produced by severe plastic deformation methods such as mechanical alloying (MA) and ion beam mixing (IBM) (which involves large displacements of atomic positions due to irradiation) do not follow phase diagram restrictions, and thus, the type of metastable phases produced are significantly different from those produced by liquid quenching or vapor quenching methods. Amorphous phases have been produced in some of these alloy systems.

Nb—Zr alloys exhibit a positive heat of mixing, the magnitude of which has been reported to be anywhere between +6 and +17 kJ/mol (O. Jin, Z. J. Zhang, and B. X. Liu, J. Appl. Phys. 78, 149-154 (1995), C. Suryanarayana and J. L. Liu, Int. J. Mater. Res. 103, 1125-1129 (2012), T. L. Wang, S. H. Liang, J. H. Li, K. P. Tai, and B. X. Liu, J. Phys. D: Appl. Phys. 41, 095310 (2008)—each incorporated by reference in their entirety). These alloys exhibit superconducting properties, and show the presence of metastable phases such as omega (A. Cavalleri, F. Giacomozzi, L. Guzman, and P. M. Ossi, J. Phys.: Condens. Matter 1, 6685-6693 (1989), G. Aurelio, A. Fernandez Guillermet, G. J. Cuello, and J. Campo, Metall. Mater. Trans. A 32, 1903-1910 (2001)—each incorporated by reference in their entirety). Jin et al. have shown that an amorphous phase could be produced in ion-irradiated multilayer thin films corresponding to the composition of Nb-40 at % Zr (O. Jin, Z. J. Zhang, and B. X. Liu, J. Appl. Phys. 78, 149-154 (1995)—incorporated by reference in its entirety). Further, through thermodynamic calculations and IBM of multilayered thin films, Wang et al. have shown that an amorphous phase could be produced in the Nb—Zr system in the wide composition range of 8-88 at. % Zr (T. L. Wang, S. H. Liang, J. H. Li, K. P. Tai, and B. X. Liu, J. Phys. D: Appl. Phys. 41, 095310 (2008)—incorporated by reference in its entirety). Most recently, Suryanarayana and Liu have shown that there were indications of formation of an amorphous phase in mechanically alloyed Nb-10 and 20 at. % Zr powder mixtures (C. Suryanarayana and J. L. Liu, Int. J. Mater. Res. 103, 1125-1129 (2012)—incorporated by reference in its entirety).

BRIEF SUMMARY

The foregoing paragraphs have been provided by way of general introduction, and are not intended to limit the scope of the following claims. The described embodiments, together with further advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings.

An object of the invention is a method for preparing an alloy, comprising mixing elemental powders of Nb and Zr to obtain a powder mixture, and mechanical alloying with a ball mill, preferably with tungsten carbide balls, to obtain an alloyed powder mixture, where the alloyed powder is more than 50% amorphous, and synthesizing a nanostructured alloy by subsequent spark plasma sintering.

The alloy is of formula Nb_(x)Zr_(y) in which x and y represent the atomic ratio of Nb and Zr atoms and x:y ranges from 40:60 to 60:40, preferably from 50:50 to 60:40. In an embodiment the method includes alloying elemental powders of Nb and Zr to obtain a composition comprising at least one alloy selected from the group consisting of Nb₅₀Zr₅₀, Nb₅₅Zr₄₅, and Nb₆₀Zr₄₀.

In one embodiment, the alloy is at least 50% amorphous. In one embodiment, the alloy comprises Nb₅₀Zr₅₀, and the alloyed powder is at least 60% amorphous, as determined by X-ray diffraction.

In another embodiment, the alloy comprises Nb₅₅Zr₄₅, and the alloyed powder is at least 56% amorphous, as determined by X-ray diffraction.

In another embodiment the alloy comprises Nb₆₀Zr₄₀, and the alloyed powder is at least 56% amorphous, as determined by X-ray diffraction.

In one embodiment of the invention, a rotational speed of the ball mill is preferably from 100 to 500 rpm. In another embodiment, a rotational speed of the ball mill is 300 rpm.

In another embodiment, the method further comprises sintering, for example, by spark plasma sintering.

In another embodiment of the invention, the mechanical alloying is performed at a temperature ranging from 20 to 35° C.

In one aspect of the invention, the powder mixtures are alloyed in tungsten carbide vials with the tungsten carbide balls.

In another embodiment, a ball to powder weight ratio during the mechanical alloying is from 8:1 to 12:1 by weight. In another embodiment, a ball to powder weight ratio is 10:1.

Another object of the invention is a Nb—Zr alloy composition obtained by the method, having an average crystallite size of 18 to 26 nm, hardness of at least 580.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing XRD patterns for developed alloy, milled for 10 hours for compositions (a) Nb₅₀Zr₅₀, (b) Nb₅₅Zr₄₅, and (c) Nb₆₀Zr₄₀.

FIGS. 2A and 2B are TEM bright-field images and selected area electron diffraction (SEAD) patterns of Nb₆₀Zr₄₀ composition milled for 10 hours.

FIG. 3 is an SEM image for the developed nanostructured alloy.

DETAILED DESCRIPTION

There are several elemental pairs that show a positive heat of mixing, thus making it difficult, if not impossible to produce alloys from them. The present disclosure is directed to production of alloys via non-equilibrium processing routes. Among systems with positive mixing enthalpy is the Nb·Zr binary system which has a value of heat of mixing of between +6 and +17 kJ·mole-1. This Nb—Zr binary system is phase separating at room temperature and pressure.

One advantage of the present invention is the ability to produce partially amorphous phase and nanostructured Nb—Zr alloy from an elemental pair that otherwise has a positive heat of mixing using a solid-state synthesis route. Amorphous phases exhibit remarkable properties over crystalline counterparts which make them promising candidates for technical applications. Moreover, nanostructured materials possess unique surface and exceptional mechanical properties, which can be considered for future generation orthopedic biomaterials.

The present invention includes an alloy composition and a method for preparing the alloy.

The developed alloy has an average crystallite size from 18 to 26 nm, preferably from 15 to 40 nm, preferably from 17.5 to 25.9 nm, preferably from 18 to 23 nm, especially preferably from 20 to 21 nm.

The alloy has hardness HV2 of at least 500, preferably at least 550, preferably from 571 to 584, preferably from 575 to 580.

The method for preparing the alloy composition includes production of amorphous material in a powder form and synthesis of nanostructured Nb—Zr alloy in a bulk form from immiscible pair elements. The method involves blending Nb and Zr powders in specific atomic percentages to give specific compositions which are milled for certain duration. Then, the alloyed Nb—Zr powders are sintered using spark plasma sintering.

The Nb—Zr alloys as used herein, have at least 60% of an amorphous phase for composition Nb₅₀Zr₅₀ and at least 56% of an amorphous phase for compositions Nb₅₅Zr₄₅ and Nb₆₀Zr₄₀ as determined by X-ray diffraction. Preferably the Nb—Zr alloys are at least 50% amorphous, preferably at least 60% amorphous.

A method for preparing the alloy comprises:

(1) mixing elemental powders of Nb and Zr to obtain a powder mixture; and

(2) mechanical alloying in a ball mill, preferably with tungsten carbide balls, to obtain an alloyed powder mixture.

In the mixing (1), the elemental powders of Nb and Zr are used as a starting material. The powders are mixed in an atomic percentage of from 40-60% to 60-40%, preferably 50-60% Nb and 40-50% Zr, based on a total of 100% Nb+Zr. Preferably, the nominal compositions are Nb₅₀Zr₅₀, Nb₅₅Zr₄₅, and Nb₆₀Zr₄₀. The mixed powder preferably includes the presence of elemental Nb and Zr only in which no other phase is present. The mixed powder preferably consists of Nb and Zr. In one embodiment the mixed powder consists essentially of Nb and Zr and excludes components that materially affect the ability of the alloying to form a partially amorphous alloy.

The time for amorphization decreases with an increase in Zr content in the powder blend. Nb has a higher melting temperature than Zr and thus the time required for amorphization increases with increasing Nb content. Furthermore, the thermal stability of the amorphous phase is expected to be higher because of the increased melting temperature of the alloy with increasing Nb content.

In the mechanical alloying (2), the process is performed using a ball mill. The rotational speed of the ball mill is preferably from 100 to 500 rpm, preferably from 250 to 400 rpm, particularly preferably from 275 to 350 rpm, and especially preferably from 300 to 325 rpm. The mechanical alloying is performed at a temperature ranging from 20 to 35° C., preferably from 25 to 30° C., especially preferably up to 28° C.

The powder mixtures may be alloyed by loading into tungsten carbide vials containing tungsten carbide balls. A ball to powder weight ratio is from 8:1 to 12:1, preferably from 9:1 to 11:1, especially preferably about 10:1. To avoid or reduce contamination, the milling is preferably carried out under an argon atmosphere cover.

Preferably, a process control agent is not employed in order to avoid contamination of blended powders.

The milling (mechanical alloying) is performed for at least 10 hours up to 70 hours, preferably from 20 to 45 hours, and especially preferably from 30 to 40 hours. On milling the powder, the intensities of the diffraction peaks of both the metals decrease and their widths increase due to a reduction in crystallite size and introduction of lattice strain. However, on continued milling of the powder blend to about 10 hours, the diffraction peaks are replaced by a broad diffuse halo, corresponding to the peak of Nb, showing that an amorphous phase has partially formed. The fraction of the amorphous phase increases with milling time, reaching about 60 volume % on milling the powder for 30-40 hours. However, on continued milling to 50-70 hours, a crystalline phase forms. Due to the milling, a phenomenon termed as mechanical crystallization (M. L. Trudeau, R. Schultz, D. Dussault, and A. Van Neste, Phys. Rev. Lett. 64, 99-102 (1990); U. Patil, S.-J. Hong, and C. Suryanarayana, J. Alloys Compd. 389, 121-126 (2005); S. Sharma and C. Suryanarayana, J. Appl. Phys. 102, 083544 (2007)—each incorporated by reference in their entirety).

The method for preparing the alloy may further comprise:

-   -   (3) sintering the alloy by spark plasma sintering.

A spark plasma sintering machine may be employed for the consolidation of the Nb—Zr alloy prepared by the mechanical alloying.

The spark plasma sintering machine (FCT system-model HP D50, Germany) was used for the consolidation of the Nb₆₀Zr₄₀ sample prepared by mechanical alloying.

The method achieves superior properties enhanced by amorphization, such as improved mechanical properties, high resistance to corrosion, high magnetic permeability, and temperature-independent electrical conductivity. Moreover, Nb—Zr alloys meet the criteria for biomaterials in terms of biocompatibility, resistance to corrosion, mechanical considerations, and ionic cytotoxicity, which makes the Nb—Zr alloys excellent candidates for biomedical applications. In addition to biomedical applications, other potential non-medical applications are possible due to the enhanced superconducting properties of the Nb—Zr alloys.

Example

Elemental powders of Nb and Zr (both −325 mesh and 99.8% metal base) were used as starting material. The powders were mixed in an atomic percentage to give the nominal compositions of Nb₅₀Zr₅₀, Nb₅₅Zr₄₅, and Nb₆₀Zr₄₀. The mechanical alloying process was performed using a ball mill (Pulverisette 5) at a rotational speed of 300 rpm at room temperature. The powder mixtures were put into tungsten carbide vials with tungsten carbide balls which satisfy the ball to powder weight ratio of 10:1. To avoid/reduce contamination the milling experiments were carried out under argon atmosphere. No process control agent was used to avoid contamination of blended powders. The spark plasma sintering machine (FCT system-model HP D50, Germany) was used for the consolidation of the Nb₆₀Zr₄₀ sample prepared by mechanical alloying.

Both X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) were used to confirm the developed amorphous phase. XRD is a simple method used to determine the presence of an amorphous phase. Presence of a sharp diffraction peak corresponds to a crystalline material, while the disappearance of sharp peaks and observation of an extremely spread diffraction peak gives indication of existence of amorphous phase. FIG. 1 shows XRD patterns for the developed alloy for compositions (a) Nb₅₀Zr₅₀, (b) Nb₅₅Zr₄₅ and (c) Nb₆₀Zr₄₀. After 10 h of milling a partial amorphous phase started appearing. The percentage of amorphization was quantified using the EVA software available with the X-ray diffractometer. This percentage was 60% amorphous for composition Nb₅₀Zr₅₀ and 56% amorphous for both compositions Nb₅₅Zr₄₅ and Nb₆₀Zr₄₀. To confirm the results obtained from XRD, transmission electron microcopy (TEM) investigation was also carried out. FIG. 2 shows the TEM micrograph and SADP pattern of milled Nb₆₀Zr₄₀ at 10 h. The presence of an amorphous phase was confirmed by the high-resolution TEM micrograph that shows the salt-and-pepper like contrast (FIG. 2(a)) and the presence of a broad diffuse halo in the diffraction pattern (FIG. 2(b)). These results coincide well with the results obtained by XRD. SEM was used to reveal structure of the developed alloy by spark plasma sintering as shown in FIG. 3.

Amorphous alloy or bulk metallic glasses have unique mechanical, physical, chemical, corrosion, and wear properties. In accordance with the invention, partially amorphous alloy was obtained from positive heat of mixing pairs Nb and Zr prepared by mechanical alloying for the first time. The alloys of this invention are at least 60% amorphous for composition Nb₅₀Zr₅₀ and at least 56% amorphous for both compositions Nb₅₅Zr₄₅, and Nb₆₀Zr₄₀. Such development will open a horizon for developing more amorphous alloys from immiscible systems that exhibit excellent properties. The present disclosure has demonstrated that the use of a non-conventional processing technique of mechanical alloying is beneficial in taking materials to new limits and forming phases that are not possible to form using conventional processing. Among the developed sintering process, spark plasma sintering was effective in obtaining a full densification in metallic materials at relatively lower temperature and in a short sintering time which prevents grain growth and minimizes the formation of undesired phases as well as eliminates the need for the pre-compaction of the powder.

The presently developed nanostructured system may be used in biomedical applications as both Nb and Zr meet the criteria of biomaterials and are biocompatible. The nanostructured alloy obtained (FIG. 3) has average pore diameter of 200 nm, the average pore diameter may otherwise vary from 50 to 1000 nm, preferably from 100 to 500 nm, preferably from 150 to 250 nm. These porous materials have potential for implant application as tissue will grow and fix in the pores.

Thus, the foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. As will be understood by those skilled in the art, the present disclosure may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present disclosure is intended to be illustrative, but not limiting of the scope of the disclosure, as well as other claims. The disclosure, including any readily discernible variants of the teachings herein, defines, in part, the scope of the foregoing claim terminology such that no inventive subject matter is dedicated to the public. 

1. A method for preparing an alloy, comprising mixing elemental powders of Nb and Zr to obtain a powder mixture consisting of Nb and Zr, and mechanically alloying the powder mixture consisting of Nb and Zr in a ball mill to form a mechanically alloyed powder of atomic formula Nb_(x)Zr_(y), where x:y represents the atomic percentage and ranges from 40:60 to 60:40, wherein the mechanically alloyed powder is more than 50% amorphous, as determined by X-ray diffraction.
 2. The method of claim 1, wherein the mechanical alloying of the powder mixture forms a mechanically alloyed powder comprising at least one alloy selected from the group consisting of Nb₅₀Zr₅₀, Nb₅₅Zr₄₅, and Nb₆₀Zr₄₀. 3-4. (canceled)
 5. The method of claim 1, wherein the alloyed powder comprises Nb₆₀Zr₄₀, and the mechanically alloyed powder is at least 56% amorphous, as determined by X-ray diffraction.
 6. The method of claim 1, further comprising sintering the alloyed powder by spark plasma sintering.
 7. The method of claim 1, wherein a rotational speed of the ball mill during mechanical alloying is from 250 to 400 rpm.
 8. The method of claim 1, wherein a rotational speed of the ball mill during mechanical alloying is about 300 rpm.
 9. The method of claim 1, wherein the mechanical alloying is performed at a temperature ranging from 20 to 35° C.
 10. The method of claim 1, wherein the powder mixture is mechanically alloyed in tungsten carbide vials with tungsten carbide balls.
 11. The method of claim 10, wherein a ball to powder weight ratio is from 8:1 to 12:1.
 12. The method of claim 10, wherein a ball to powder weight ratio is about 10:1.
 13. The method of claim 1, wherein the mechanical alloying is performed for at least 10 hours up to less than 50 hours.
 14. The method of claim 13, wherein the mechanical alloying is performed for 30 to 40 hours.
 15. The method of claim 1, wherein the mechanical alloying is carried out with tungsten carbide balls. 16-18. (canceled)
 19. The method of claim 6, wherein the alloy prepared by spark plasma sintering has an average crystallite size of 18-26 nm.
 20. The method of claim 6, wherein the alloy prepared by spark plasma sintering has a hardness of at least
 500. 21. The method of claim 1, wherein the mechanically alloyed powder is from 50% to 60% amorphous, as determined by X-ray diffraction.
 22. The method of claim 21, wherein the mechanically alloyed powder comprises Nb₅₀Zr₅₀, and the alloyed powder is 60% amorphous, as determined by X-ray diffraction.
 23. The method of claim 21, wherein the mechanically alloyed powder comprises Nb₅₅Zr₄₅ or Nb₆₀Zr₄₀, and the alloyed powder is 56% amorphous, as determined by X-ray diffraction. 